From Beyond is not OP...it's a rare. Oblivion Sower and Ulamog are not OP...they are Mythics.
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Most people's issue with Moss is that it is a common, it ramps while it slows down the opponent, and on occasion is shuts the victim of the Moss out of the game.
There are two points worth responding to here.
Before that though, telling me oblivion sower isn't op is ignoring the context. The entire point was that it's 1 card in 60 and thus not that relevant.
I would argue that From Beyond and Ulamog ARE op. This is before we get to other things they can cast turn 5 like
Gaea's Revenge. And the reasons they are so safe against aggro decks.
From Beyond is a two of that tutors Ulamog, meaning it's your second and third copy of Ulamog in a deck that WILL have the mana to cast Ulamog. It also makes a token every turn and has the ability to ramp "for free". This, by itself, makes it one of the best 4 drops in the game for
decks.
Drawn early in the game, you now have a bunch of ramp and chump blockers depending on what you need. Later in the game it's going to get Ulamog. It's a freaking swiss army knife in a deck that's already far above average.
This is before we talk about synergy with cards like
Evolutionary Leap. It's way more powerful on average than Acid Moss, and thus much much more of a problem because it drives the consistency of the deck WAY up, which is much much more valuable than the 1 in 4 (being generous) games where acid moss's land destruction is relevant.
Ulamog......... the thing wins in 3 turns, has indestructible, and exiles two things upon entering the battlefield. The current best counter to Ulamog is....... Ulamog. In decks that have so much draw and thinning and tutoring that they ARE GOING TO GET ULAMOG. It's inevitable and they can even recur it if it's not exiled. The value on this one card is insane and it's completely op for the current meta.
By comparison, acid moss has to be drawn early or it's just "bad" ramp. Even then, a large number of decks, especially aggro decks, can just ignore the effect altogether.
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Responding to the second sentence, yes, most people's issue with Acid Moss is that it's common and occassionally shuts people out of the game. But IT'S NOT A GOOD REASON. People are way over-blaming it instead of looking at the things that actually make ramp an "op" deck. Things such as it's massive thinning and tutoring (compared to the rest of the meta), insane amounts of safety vs a lot of aggro, and insane counter cards like
Gaea's Revenge that make it a generally "too safe" pick in the current meta. Acid Moss is just the scapegoat people blame the way some people in HS blame
Thoughtsteal for everything.